Study suggests Earth survived near miss

Thursday, 20 October 2011 Stuart GaryABC

A billion tonne fragmenting comet which looks similar to this Hubble image of Schwassmann-Wachmann 3 may have almost hit Earth in 1883(Source: NASA, ESA, H. Weaver (APL/JHU), M. Mutchler and Z. Levay (STScI))

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Dodged a bullet A new study claims life on Earth was almost wiped out by a comet in 1883.

The findings are based on a re-examination of a report originally published in the French journal L'Astronomie in 1886.

According to the article, Mexican astronomer Jose Bonilla, while working at the Zacatecas observatory in August 1883, spotted a group of about 450 objects, each surrounded by mist, passing in front of the Sun.

Bonilla didn't have an explanation for what he observed and L'Astronomie's editor suggested, it may have been nothing more than birds, insects or dust.

Speculation about the origins of these objects has intrigued people for years with suggestions ranging from atmospheric conditions to UFOs.

The new hypothesis by scientists including Hector Manterola at the National Autonomous University of Mexico claims the misty appearance of the objects indicates they were fragments of a comet which had broken up.

Reporting on the pre-press website arXiv.org, Manterola and colleagues claim their calculations indicate the cometary fragments ranged from less than 50 metres wide, up to more than a kilometre across.

They say the progenitor comet would have been the size of Halley's comet with a mass of a billion tonnes.

As Bonilla's account is the only report of these objects, Manterola and colleagues believe the fragments were very close to Earth, otherwise other observatories would have also detected them as they passed in front of the Sun.

Using parallax calculations, they placed the fragments at between 600 and 8000 kilometres above Earth.

When combined with trajectories of the fragmented comet's path Manterola and colleagues believe the observations best fit in with the Pons-Brooks comet detected two years later from the Cincinnati Observatory.

In 1908 an asteroid or comet a few tens of metres across, airburst over the Tunguska region of Siberia, flattening some 80 million trees over an area of 2000 square kilometres.

Based on the damage caused at Tunguska, Manterola and colleagues conclude, had these cometary fragments hit Earth, they would probably have resulted in an extinction event.

Hard not to notice

"If a comet as big as they're saying came that close to Earth, it's amazing no one else saw it," says Francis. "It beggars belief that no one else noticed 400 comets that bright whizzing around over several days."

According to Francis the odds of that happening and not being noticed "would be staggering".

"I think the editor of the original article in the magazine was probably correct, the objects were birds, insects or dust clouds."